Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data communication. In general, a wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available radio resources. Examples of multiple access systems include a time division multiple access (TDMA) system, a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc.
In a wireless communication system, one carrier is considered in general even if a bandwidth is differently set between an uplink and a downlink. In the 3rd generation partnership project (3GPP) long term evolution (LTE), one carrier constitutes each of the uplinks and the downlinks on the basis of a single carrier, and the bandwidth of the uplink is symmetrical to the bandwidth of the downlink. However, except for some areas of the world, it is not easy to allocate frequencies of wide bandwidths. Therefore, as a technique for effectively using fragmented small bands, a spectrum aggregation technique is being developed to obtain the same effect as when a band of a logically wide bandwidth is used by physically aggregating a plurality of bands in a frequency domain. The spectrum aggregation includes a technique for supporting a system bandwidth of 100 mega Hertz (MHz) by using multiple carriers even if, for example, the 3GPP LTE supports a bandwidth of up to 20 MHz, and a technique for allocating an asymmetric bandwidth between the uplink and the downlink.
A UE (User Equipment) can have a different capability for the reception of downlink carriers or the transmission of uplink carriers. For example, one UE may only support an aggregation of uplink/downlink carriers of 20 MHz or less while another UE may only support an aggregation of uplink/downlink carriers of 100 MHz or less. Because of such a restriction, a BS (Base Station) needs to determine how an aggregation of carriers should be established when a specific UE first performs communication with the BS.